Surface Oceans Around Distant Stars

by Paul Gilster on April 28, 2008

Would large amounts of water on the surface provide a glint of light in both the infrared and visible spectrum if we study a distant exoplanet long enough? That’s the premise of an investigation now in progress, one aiming to find Earth-like planets in the habitable zone of a star. Darren Williams (Penn State Erie) and Eric Gaidos (University of Hawaii) have something more in mind than analyzing a planetary atmosphere for signs of water. They want to spot planets with water on the surface.

If the goal sounds chimerical now, bear in mind that various planet-hunting missions like Terrestrial Planet Finder (in its various incarnations) and Darwin are being designed to allow direct observation of planets as small as the Earth. Such observatories, which may be in place within two decades or less, could also examine the visible and infrared light curve of such planets over the course of an entire orbit.

“We are going to look at the planets for a long time,” says Williams. “They reflect one billionth or one ten billionth of their sun. To gain enough light to see a dot requires observation over two weeks with the kinds of telescopes we are imagining. If we stare that long, unless the planet is rotating very slowly, different sides of the planet will come through our field of view. If the planet is a mix of water, we are going to see the mix travel around the planet.”

According to the paper on this work, half of all detected extrasolar planets will have orbital inclinations that make it possible to detect surface oceans. When we looked at this idea back in January, one thought particularly stood out from the team’s paper: “… of all the extremely difficult measurements astronomers hope to make with a TPF-class telescope, time-series photometry and polarimetry that can lead to the identification of specular reflection from surface water might be the easiest.” This Penn State news release catches up with the current thinking of the researchers.

What to do while we wait for a mission capable of making such detections? One idea is to do what Voyager did long ago in the famous ‘Pale Blue Dot’ photograph, looking back at our own planet to study its signature at various wavelengths. Voyager’s view was fascinating largely for what it represented — our world from the outer reaches of the Solar System — but Williams has now enlisted the Mars Express and Venus Express missions to occasionally view the Earth and examine its various phases. That data should provide a useful baseline for the kind of studies that may one day find Earth’s twin.

Image: The famous ‘pale blue dot’ photograph taken by Voyager 1. Looking back at our planet from distant viewpoints will tell scientists much about detecting water on distant worlds. Credit: NASA/JPL.

And if we do find a world in the habitable zone with liquid water on the surface? The odds for life will obviously go up, supplemented by whatever spectrographic data we gather. As to intelligence, we all have our viewpoints on its likelihood, but such observations may prove inconclusive. About the only certainty we’ve had in our explorations of our own Solar System is that we have continued to be surprised, so it’s reasonable to assume we’ll run into more than our share of enigmas around other stars.

The paper is Williams and Gaidos, “Detecting the Glint of Starlight on the Oceans of Distant Planets,” upcoming in Icarus and available online. Let me also recommend Greg Bear’s Queen of Angels for a look at just how tantalizing (and confounding) the detection of exoplanetary life can be, in this case by a probe in the Centauri system.

Dectection of ocean water by spectroscopic light analysis is a brilliant idea. Perhaps detection of frequencies that would betray the presence of chlorophil based plant life or similar vegitation-based land surface cover would be outstanding.

I have a question. Is the presence of chloropyll in the water taken into account? The Earth’s oceans contain a significant amount of it in the form of plankton, and I’m sure there’s enough of them to skew the reading off the “pure pale blue dot” we’d expect from these observations. I’m not sure if the Voyager image is of high enough resolution, but I’ll bet a detailed spectrographic analysis would bear this out.
And when you consider the fact that chloropyll can come in lots of other colors.. pink water ocean anyone? *grins*

In a decade or so we’ll have the technology to detect Earth-sized planets. I think that this will greatly increase the interest in an interstellar mission because that will be the first time that we will have a specific planetary target for an interstellar mission.

Hopefully one of the Alpha Centauri stars will have a Earth-sized planet with continents and warm oceans. But I’m not holding my breath. More likely we’ll find ourselves in the scenario where we have several interesting target planets with the most interesting ones being further out than the closer ones. So the choice of a target for an interstellar mission might be a tough one.

Making the decision worse would be that we probably won’t be able to realistically get an adequately sized craft up to very high speeds (e.g. .5c) but only .1c if we’re lucky. So we’d probably expect to have to wait at least 47 years to get our close-up picture of a nearby planet and 200 or so years to get a close-up picture of the planet we’re most interested in. Yet in those time frames we’ll be able to dramatically increase our ability to view smaller and smaller planets and even moons. And we’ll also be increasing the speed of our interstellar craft.

We are just now getting a somewhat good look at the moons around Jupiter and Saturn. And yet we now have had several missions to Mars and have even developed the beginnings of an integrated system of rovers, reconnaisance, and communication satellites. I think that part of the reason for this is because the human colonization of Mars is a real possibility and so these things are steps towards that goal.

So our choice for an interstellar discovery mission may in fact be driven by the potential for a follow-up colonization mission. Especially this would be true considering the technologic development that could occur during the 47 to 200 year time-lag from launch of the discover mission to the launch of a colonization mission.

With these possibilities I wonder what sort of criteria for we would therefore choose for our first interstellar discovery mission.

I think people will argue over whether we should “infest” another biosphere with our supposedly odious presence.

I suspect the push will be to send explorers, presumably uploads, with no intention of colonisation. An orbiter mission can receive new crew as uploads via data-transmissions from Earth and can use free-space resources to build probes and an industrial base to avoid damage of the planet being studied. Colonisation always sounds cool to fleshers, but is elitist at the Earth end of things, and ecocidal at the other end.

Ponder Isaac Asimov’s “Spacer Worlds” and his early short story “Mother Earth” – I think he hit the nail on the head about the elitist and eugenicist attitudes that would dominate early colonisation efforts. By no stretch of the imagination could a substantial fraction of the terrestrial population be launched to another star system, so it will always be an ideologically driven exercise with some idea of “purity” controlling who goes and who stays.

All that assumes we remain fleshers and there’s no Greg Egan style “Introdus” into virtual worlds if uploading is easy and desirable. Even if we remain flesh there will be pressure to develop “body-scanning” and reconstitution of bodies at the destination.

But that’s just my philosophical prejudices. I probably can’t say what the bulk of humanity might choose – if we get a choice.

In a decade or so we’ll have the technology to detect Earth-sized planets. I think that this will greatly increase the interest in an interstellar mission because that will be the first time that we will have a specific planetary target for an interstellar mission.

I don’t think we’re going anywhere until we can get there within a lifetime — either through very fast ships, hibernation, or extended life spans. Perhaps, as you say, if the Centauri system has a promising target, then it would spur an interstellar scouting mission, but if all the promising targets are more than a dozen of so light years away, I suspect it will be at least a couple of centuries before we venture forth from our solar system.

Consequently, I suspect the boom in spacecraft building will come in the form of bigger and better TPF’s. One nth generation TPF has the chance of imaging dozens of exo-Earths (hundreds?, thousands?), whereas one interstellar probe will take decades to survey just one system.

I expect we will expand out of the solar system eventually, but unless some kindly alien neighbors drop in with a short cut to the stars, (any Vulcans hiding out there?) I think we’re looking at centuries rather than decades before that happens.

About the streak in the ‘pale blue dot’ image, NASA says this: “From Voyager’s great distance Earth is a mere point of light, less than the size of a picture element even in the narrow-angle camera. Earth was a crescent only 0.12 pixel in size. Coincidentally, Earth lies right in the center of one of the scattered light rays resulting from taking the image so close to the sun.” A NASA caption I saw elsewhere notes that there are a number of instrumental artifacts elsewhere in the image due to the high magnification.

tacitus,thank you you said exactly what had been forming in my mind as i looked up your comment.in my opinion finding an earth like planet will be a huge shot in the arm for the space program.why people will be SCREAMING to go there! and probably people with no concept whatsoever about interstellar flight and who 5 minutes ago thought that space was a waste of money!!? yet…lol the renewed interest will be great in spurrung the program on. even if only by getting new people interested,having studies articles in various papers and magazines etc. you all get my point.thank you very much george ps one more thing please – i read recently that stephen hawking had said that he thinks that starships are about 200 to 500 years off.just for the record i had said by sheer coincidence about two weeks ago that i thought we would have a manned starship by 2175. “only” 167 years off.fits nicely with the low end of his estimate. just my opinion but i look at it this way- in just 100 years look at the progress which has been made.i mean planes that used to go 130 mph now do 1500!!! etc etc so i felt that in way over 100 more years at the rate we are seeing progress in science and technology,well, it just seems likely.would like to hear everybody elses ideas too. all the best again, george

There is sparse dust located in the solar system’s plane that is dimly visible from Earth if the sun is blotted out. It’s responsible for the Zodiacal light, which is sunlight scattering off that dust. Since the Earth is in crescent phase that would put the sun just off the picture frame, indicating that this may be the cause. I’m only speculating; it could merely be an artifact.

Yes, I also strongly agree with tacitus: for the coming decades, maybe even centuries, further exploration of other planetary systems will be in the form of (greatly) improved telescopes and the like, for cost/benefit reasons.

The only early incentive for probes I can think of would be the discovery of a truly exceptionally attractive earthlike planet around Alpha Centauri, either life-bearing or so biocompatible that it is (almost) ready for settling.

ronald,the exceptionally attrractive earthlike world you mention above would certainly be a bonanza for the development of space craft but other wise i think you are correct. but think of this: the greatly improved telescopes of which you speak just confirm how great said planet is! then i myself will be screaming for the development of starships!!! no kidding. lets all keep our fingers crossed.thank you very much, your friend george

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In Centauri Dreams, Paul Gilster looks at peer-reviewed research on deep space exploration, with an eye toward interstellar possibilities. For the last nine years, this site has coordinated its efforts with the Tau Zero Foundation, and now serves as the Foundation's news forum. In the logo above, the leftmost star is Alpha Centauri, a triple system closer than any other star, and a primary target for early interstellar probes. To its right is Beta Centauri (not a part of the Alpha Centauri system), with Beta, Gamma, Delta and Epsilon Crucis, stars in the Southern Cross, visible at the far right (image: Marco Lorenzi).

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